Apparatus and method for controlling a forced flow once-through steam generator



June 1962 w. H. ARMACOST 3,038,453

APPARATUS AND METHOD FOR CONTROLLING A FORCED FLOW ONCE-THROUGH STEAM GENERATOR Filed Feb. 7, 1957 4 Sheets-Sheet 1 K E 5?; FIG. 3 3 DJ E I00 STEAM LOAD I. OF MAXIMUM LOAD a 5 SA E o ,1, STEAM LOAD 'l. OF MAXIMUM LOAD ,a g FIG. 5

STEAM LOAD In OF MAXIMUM LOAD FIG. 2

INVENTOR WILBUR H. ARMACOST WAQZ/ ATTORNEY June 12, 1962 w. H. ARMACOST 3,038,453

APPARATUS AND METHOD FOR CONTROLLING A FORCED FLOW ONCE-THROUGH STEAM GENERATOR Filed Feb. 7, 1957 4 Sheets-Sheet 2 WILBUR H. ARMACOST ATTORNEY J n 1962 w. H. ARMACOST 3,038,453

APPARATUS AND METHOD FOR CONTROLLING A FORCED FLOW ONCETHROUGH STEAM GENERATOR 4 Sheets-Sheet 5 Filed Feb. '7, 1957 INVENTOR WILBUR H. ARMACOST way ATTORNEY June 12, 1962 w. H. ARMACOST 3,038,453

APPARATUS AND METHOD FOR CONTROLLING A FORCED FLOW ONCE-THROUGH STEAM GENERATOR Filed Feb. '7, 1957 4 Sheets-Sheet 4 LOAD CIRCUIT INVENTQR WLLBUR H. ARMACOST ATTORNEY nite States atent 3,038,453 APPARATUS AND METHOD FOR CONTROLLING A FORCED FLOW ONCE-THROUGH STEAM GENERATOR Wilbur H. Armacost, Scarsdale, N.Y., assignor to Combustion Engineering, Inc., New York, N.Y., a corporation of Delaware Filed Feb. 7, 1957, Ser. No. 638,826 12 Claims. (Cl. 122-406) This invention relates to steam generating apparatus of the forced flow once-through type and modifies in a novel manner the once-through system by recirculating a portion of the working fluid within the fluid heating sections of the flow circuit. More specifically the invention When applied to a forced flow steam boiler of the once-through type provides a novel apparatus and method of control by increasing the amount of working fluid recirculated through the fluid heating sections when the steaming load on the boiler is decreased, and by decreasing the amount of working fluid recirculated when the steaming load on the boiler is increased. It further provides a novel apparatus and method for independently controlling the amount of fluid recirculated in specifically selected fluid heating sections in both the liquid phase and the vapor phase of the vapor generating and vapor heating cycle of a once-through forced flow steam generator.

THE PROBLEM ARISES OUT OF lNCREASE IN STEAM PRESSURE Power plant designers and equipment manufacturers are constantly striving to reduce the production cost of electric power. Since the investment cost of the power plant itself is an important component of the overall production cost of electricity, a reduction in the plant net heat rate is equally important to a reduction in the cost of any new power plant.

Up to now the most effective means for decreasing the net heat rate has been an increase in operating steam pressures and temperatures. With a mounting rise in steam pressures the boiler feed pump assumes a rapidly growing importance in the original and operating costs of modern steam power plants. Some years ago when steam turbines operated at the modest pressure of 625 p.s.i. (pounds per square inch) the boiler feed pump consumed only 0.5% of the total plant output. At todays pressures of 2350 to 5500 p.s.i. the boiler feed pumps share of the total planned capability has grown to 2.5% and 6% respectively. Consequently the amount of power consumed by the boiler feed pump has become an important factor in the economic valuation of steam cycles operating at high pressures, especially in boilers of the forced fiow once-through type.

This increase in feed pump power requirement is primarily due to the high velocities needed in the boiler tubes and superheater tubes to keep the tube metal temperature within safe limits, and the attendant high pressure losses. Thus in some designs of steam generators of the once-through type it has been advantageous to locate the water heating surface to the furnace around the burners so as to make it possible to operate with a low metal tube temperature. In other designs it has been desirable to place the water heating surfaces primarily in the convection passes of the steam generator and utilize the furnace walls to accommodate steam superheating and reheating surfaces.

Due to the fact that these boilers must at times deliver a steaming capacity as low as one-third or less of maximum capacity, it is necessary to have suflicient velocities through the heated tubes at this low load so as to obtain uniform fluid distribution and heat absorption in the various parallel fluid heating tubes. It therefore ice becomes necessary that these flow circuits are designed for the minimum safe velocities through the tubes at the lowest steam generating load. When operating the boiler at the maximum load, however, the velocity through the fluid heating tubes increases in proportion to the steaming load and the pressure drop roughly in proportion to the square of the load ratio. Thus it is found, for example, that whereas the pressure drop at 30% load may be 15 to 20 p.s.i. through the water heating section, it may increase at top load to to 200 p.s.i. Proportional increases of course also occur in the economizer section and the superheating section. Consequently the difference in rated discharge pressure required at the boiler feed pump andthe pressure maintained at the turbine inlet may reach a value as high as 600 p.s.i. and even exceed this value in once through type boilers operating at supercritical steam pressures.

Reducing this pressure difference and the high feed pump power requirements resulting therefrom has become one of the important problems arising out of the steady increase in operating pressure.

THE PURPOSE AND OBJECTS OF THE INVENTION In accordance with my invention the above conditions are greatly improved by providing in a once-through bo ler means for controllably recirculating the working fluid from the outlet end of the fluid heating tubes where these tubes leave the heating chamber back to the inlet end of the fluid heating tubes at the point where these tubes enter the heating chamber. Such improvements include the important feature of establishing separate and independent recirculation circuits in the various heating sect ons exemplified by water heating and superheating sections. Additionally a single recirculation circuit may also be included which encompasses the entire heating system of the once-through forced flow cycle. It further includes a control system which provides for es tablishing recirculation in any one of these heating sectlons or in any combination of two or more, in response to impulses governed by fluctuations in steam load.

In this manner the feed water pump when operating at maximum load can be designed for a pressure drop through the fluid heating sections thereof of only a fraction of that which would be required in a once-through system that is not equipped with my inventive improvement. When the unit is operating at lower loads than maximum load :1 complementing amount of working fluid 18 then recirculated to maintain the velocity in the tubes at or above the safe velocity.

It is accordingly a main object of the invention to sub stant ally reduce the design operating head and the power requirement of the feed water pump by reducing the pressure drop through the fluid heating system.

It is another object of the invention to protect the Water heating tubes exposed to the hot gases against overheating when operating at low loads.

Still another object of the invention is to control the flow of working fluid through the heating tubes so as to keep the pressure drop therethrough substantially constant throughout a selected load range, by increasing the quantity of fluid recirculated through these heating tubes with a decrease in steam load, and decreasing the quantity of fluid recirculated therethrough with an increase in steam load.

A further object of the invention is to lower the safe steam load at which the boiler can be operated, thereby increasing the overall capacity range of the steam gen-' erator.

several heating sections of a forced flow once-through fluid heating cycle, and providing novelly flexible controls therefor to meet changing operating conditions caused by fluctuations in steam load and changes in heating gas temperatures.

Other objects and advantages of the invention will become apparent from the following description of illustrative embodiments thereof when taken in conjunction wtih the accompanying drawings wherein:

FIGURES 1 and 2 are diagrammatic representations of a forced flow once-through steam generating power plant modified with the herein disclosed inventive improvement; FIG. 1 showing a system of several recirculation circuits and FIG. 2 a recirculation circuit for the water heating section only.

FIGURES 3, 4 and are graphs showing the pressure drop through the water heating circuits vs. the steam load for a modified once-through steam generator equipped with a recirculating circuit superimposed upon the water heating tubes in accordance with my invention and as illustratively shown in FIG. 2.

More specifically FIG. 3 graphically indicates conditions prevailing when the circulation pump is operating from zero to 30% load, FIG. 4 from zero load to 50% full load and FIG. 5 from zero load to full load.

FIGURE 6 is a diagrammatic representation of my control system applied to a steam generator designed for forced flow modified once-through operation within the supercritical pressure range, i.e. at a pressure at or in excess of 3206 p.s.i. abs. (pounds per square inch absolute), and which in accordance with my invention has a recirculation circuit superimposed upon the water heating tubes, the flow therethrough being controlled by fluctuations in pressure drop through said tubes or fluctuations in steam load.

FIGURE 7 is a diagrammatic representation of a steam generator of the forced flow modified once-through type operating in the subcritical pressure range, i.e. below a pressure of 3206 psi. abs, and having a recirculation circuit superimposed on the water heating tubes, the flow therethrough being controlled by varying the rotational speed of the circulation pump in response to variations in feed pump revolutions.

' FIGURE 8 is a diagrammatic representation of a steam generator of the forced flow modified once-through type operating in the supercritical pressure range, and having in accordance with my invention a recirculation circuit superimposed upon the water heating tubes, the flow therethrough being controlled by a throttle valve in response to turbo-generator power output, or by the steam input to the recirculating pump drive.

A BIRDS EYE VIEW OF THE PROBLEM AND THE SOLUTION In FIG. 1 there is illustrated in diagrammatic form the basic circuit of a forced flow modified once-through steam generator power plant operating in the supercritical pressure range. A feed pump F takes water from a water tank W and delivers this water under pressure to an economizer E wherein the water is heated to near saturation temperature. From economizer E the water flows through additional water heating tubes H which may serve as a lining for the walls of the boiler furnace or may constitute convection heating surface exposed to the combustion gases after they have left the furnace. In tubes H transition takes place from the liquid phase to the vapor phase of the working fluid before the steam is conducted to superheater S wherein the steam is superheated to a final temperature of 1100 F., for example. Upon leaving the superheater S the steam is conveyed to a point of use such as turbine T where the thermal energy of the steam is converted into mechanical energy for driving an electric generator G. The steam exhausted from the turbine T is condensed in condenser C and returned to water tank W by way of condenser pump P.

My inventive contribution to this basic cycle comprises the application and control of recirculation circuits superimposed upon selected heating sections as well as upon the entire once-through vapor generating and vapor heating system. Thus pump R may take working fluid from the outlet of superheater S and return it to the inlet of economizer E. Pump R may take working fluid from the outlet of economizer E and return it to the inlet thereof. Pump R may take Working fluid from the outlet of water heating tub es H and return it to the inlet thereof. And pump R may take working fluid from the outlet of superheater S and return it to the inlet thereof. A flow measuring device 57 responsive to fluctuations in steam output of the steam generator, transmits impulses to regulator 61 by way of flow indicator 58 for operating valve 54 controlling the amount of working fluid being circulated.

How my herein disclosed improvement accomplishes the basic object of the invention as well as other objects earlier herein set forth, will now be described by reference to FIG. 2 and to the graphs shown in FIGS. 3, 4 and 5.

In FIG. 2 there is illustrated in diagrammatic form the basic circuit of a forced flow modified once-through steam generator power plant operating at sub-critical pres-sure. For simplicity sake only one recirculation circuit is here illustratively shown as being superimposed upon the water heating section represented by tubes H.

Similar to earlier description in connection with FIG. 1 a feed pump F takes water from a water tank W and delivers this water under pressure to an economizer E wherein the water is heated to near-saturation temperature. From economizer E the water flows through additional water heating tubes H which may serve as a lining for the walls of the boiler furnace or may constitute convection heating surface exposed to the combustion gases after they have left the furnace. In tubes H transition from water to steam takes place so that when the Working fluid leaves tubes H only a residual amount of moisture is present which is separated from the steam in separator D before the steam is conducted to superheater S. In a steam generator operating in the supercritical pressure range, such as that shown in FIG. 1, the separator D is eliminated and the working fluid flows directly to superheater S wherein the steam is superheated to a final temperature and is thereupon conveyed to turbine T driving electric generator G. The steam exhausted from turbine T is condensed in condenser C and returned to water tank W by means of pump P.

As in FIG. 1, my inventive contribution to the basic cycle comprises the application and control of recirculation circuits superimposed upon fluid heating sections of this cycle. In FIG. 2 only one such recirculation circuit X, including control elements 57, 61, 58 and 54, is shown as being superimposed upon the water heating tubes H at the points A and B for example, and taking a selected portion of the working fluid from the circuit at A and returning this portion to the circuit at B by means of a recirculation pump R.

How this single recirculation circuit X contributes materially to a reduction in pressure drop and feed pump power requirements will now be described with reference to FIGS. 3, 4 and 5. Equal and additional benefits can be achieved when utilizing other recirculation circuits applied to economizer E or superheater S or more than one circulating circuit such as are shown in FIG. 1. Operating conditions will dictate which recirculation circuit or how many should be used for most economic steam generator design and operation.

Changing operating conditions are primarily reflections of fluctuations in steam load. T-hus FIG. 3 illustrates the conditions prevailing when the recirculation circuit is in operation within a load range between zero load and 3G% load; FIG. 4 illustrates the conditions prevailing when the circulation circuit is in operation within a load range extending from zero load to 50% load; and FIG. 5, when the circulation circuit is in operation throughout the 5 entire load range. Obviously load ranges other than 30% or 50% could be selected.

In FIG. 3 the solid line curve K illustrates the increase in pressure drop in the water heating tubes H (FIG. 2) between points A and B, when plotted against a rise in steam load. It is assumed for example that at 30% of maximum steam capacity the velocity through water heating tubes H, to prevent overheating, must be such as to cause a pressure drop of 20 p.s.i. This is indicated by point a in FIG. 3. Continued operation below this point would ordinarily result in a lower velocity and lower pressure drop, as shown by the dotted line K and would accordingly be unsafe. With my novelly controlled recirculation circuit superimposed upon the water heating tubes H, the minimum velocity and pressure drop of 20 p.s.i. can be maintained at even lower loads than 30%. This is indicated by straight solid line L extending from point a to zero load. Thus the quantity of water delivered by the feed pump F diminishes with the steam load as indicated by area b. On the other hand in accordance with my invention the quantity of the working fluid recirculated by recirculating pump R increases with a drop in steam load as indicated by area 0.

Accordingly the limitation heretofore imposed upon low load operation by possible overheating of tubes is entirely removed in a modified once-through steam generator that is equipped with my controlled recirculation circuits.

it will be noted in FIG. 3 that with a safe pressure drop of 20 p.s.i. (to prevent tubes H from overheating) being maintained at loads of 30% of maximum and less, the pressure drop steeply rises with loads higher than 30% and reaches a peak value of approximately 200 p.s.i. at maximum load. To reduce the high discharge pressure required at the boiler feed pump F my invention provides in addition for an apparatus and method to recirculate and control a selected portion of the working fluid during operation of the boiler above the minimum load such as between zero load and 50% load as illustrated in FIG. 4, or throughout the entire boiler load range as illustrated in FIG. 5.

Thus in FIG. 4 the pressure drop curve M begins at point d with 20 p.s.i. pressure drop maintained at 50% load and rises only to approximately 80 p.s.i. at maximum load, a saving of 120 p.s.i. over operating conditions illustrated in FIG. 3. This saving of 120 p.s.i. results in a reduction of required discharge pressure at the boiler feed pump by a similar amount and accordingly permits a substantial saving in feed pump power. Under the operating conditions illustrated in FIG. 4 the circulating pump R (FIG. 2) maintains the minimum safe pressure drop in the water heating tubes H by controlled recirculation of working fluid below the 50% load point. This is illustrated by line (FIG. 4). The delivery rate of the recirculating pump R as indicated by area e would then complement the delivery rate of feed pump F as indicated by dotted line M1 and area f.

In FIG. 5 an operating condition is illustratively shown wherein full advantage is taken of my herein disclosed inventive recirculation circuit. Here the circulating pump is in operation throughout the entire load range resulting in a lowering of the required discharge pressure at the feed pump of approximately 180 p.s.i. for the conditions assumed. Barring other limiting factors a pressure drop of 20 p.s.i. between points A and B can theoretically be maintained throughout the entire boiler load range from zero load to maximum. This is indicated by line U. The portion of the pressure drop assignable to feed pump F is indicated by dash line Q and the area h below said line. The pressure drop portion assignable to recirculating pump R and complementing that of feed pump F to make up the desired constant pressure drop of 20 p.s.i., is indicated by the area 2. above said dash line Q.

Thus it can be seen that the discharge pressure required at the boiler feed pump F can be reduced by a substantial amount by applying the principle of my invention to a once-through steam generator operating at steam pressures in the neighborhood of or exceeding supercritical pressure. Such reduction in discharge pressure results in considerable savings in cost of pump, hydraulic coupling and driver in addition to improving the efiiciency of the steam cycle due to reduction in pump power requirements.

Various Ways and means can be employed in regulating the amount of working fluid recirculated through tubes H or through other heating sections such as superheater S or economizer E in complement with the amount of working fluid delivered by the feed pump for various steam generator loads, so as to keep the pressure drop through the fluid heating tubes constant at a safe value throughout the load range. Such ways and means include regulation in response to the pressure differential between, for example, points A and B. Or such ways and means include regulation in response to steam flow to the turbine, feed water flow, air flow, fuel input, feed pump speed and turbo-generator power output. In fact in accordance with my invention any fluctuations of steam, water, air, pressure, temperature, power output, power requirement or steam consumption of feed pump etc., which affects or reflects the amount of working fluid passing through the once-through fluid heating sections of the steam generator as exemplified by water heating tubes H will be suitable to furnish an impulse for regulating the amount of working fluid to be recirculated to maintain constant the velocity or pressure drop between selected points (such as A and B for example) throughout a predetermined load range. Some of these regulating systems will now be described. For the sake of simplicity these systems illustrate only one recirculation circuit as applied to water heating tubes H. It is understood that the same or similar principles of control could be applied to more than one recirculation circuit or to recirculation circuits super-imposed upon other heating sections such as superheater S or economizer E of the oncethrough vapor generating and heating system.

Description of the FIG. 6 Embodiment In FIG. 6 there is illustrated in a diagrammatic form. a forced flow modified once-through steam generator in which the regulation of the recirculated working fluid is accomplished by response to steam flow or pressure differential.

The feed pump 10 driven by a prime mover 12 receives feed water from a water source 14 by way of conduit 16 and delivers this feed water via conduit 18 to a header 20. A throttle valve 22 is provided in conduit 18 to control the flow of the feed water. The steam generator comprises a furnace chamber 24 fired by burner 26 receiving fuel through fuel pipe 28 and air from a source not shown. A throttle valve 30 is provided in fuel pipe 28 to control the amount of fuel being burned in furnace 24.

The feed water or working fluid leaves header 20 and passes through a plurality of parallelly arranged water heating tubes 32 forming radiant and/ or convection heating surfaces exposed to the heat of the hot gases. As illustratively shown in FIG. 6 these tubes 32 may form water wall linings within furnace chamber 24. Tubes 32 upon leaving the furnace chamber enter header 34. Sufficient heating surface is provided by tubes 32 so that saturated steam is: delivered to header 34 from whence the steam then flows via conduit 36 to superheater inlet header 38, through superheater tubes 40 and into superheater outlet header 42. A steam pipe 44 delivers the steam thus superheated to a point of use such as a turbine (not shown).

In accordance with my invention a recirculating circuit is established between the outlet header 34 (at the point where the fluid heating tubes 32 are shown to leave the furnace chamber 24) and the inlet header 20 (at the point where the fluid tubes 32 are shown to enter the furnace chamber 24). This circuit comprises conduit 48 leading from header 34 to the inlet of circulating pump 50 driven by prime mover 51, and conduit 52 leading from the outlet of pump 50 to the feed water inlet header 20. A throttle valve 54- is provided in conduit 52 to control the flow of working fluid being recirculated by circulating pump 50 as later herein described.

In operation the fuel supply and feed water supply of the steam generator can be controlled by any conventional combustion control system. For simplicity sake the fuel supply is here shown as being controlled by the steam pressure and the feed water supply by the steam flow. It is understood however that other and more elaborate combustion control systems are and may be used in conjunction with the herein disclosed invention.

Referring again to FIG. 6 pressure sensing device 55 translates any pressure fluctuations resulting from a change in steam load and received from header 42,, and sends a corresponding impulse to fuel valve regulator 56, for adjusting the supply of fuel to the burner 26 and air in response to steam load in a manner well known in the art.

A flow measuring device 57 installed in the steam line 44 establishes a pressure drop which is indicative of steam flow measured by steam flow indicating device 58. This device transmits variations in steam flow by way of conduit 59 to feed water regulator 60, which in turn activates feed valve 22 thereby regulating the feed water supply in response to steam flow.

The amount of working fluid being recirculated through the fluid heating tubes 32 is here shown as being controlled by valve 54 actuated by a regulator 61 which is capable of receiving a control impulse from two selective sources. One source is the steam flow indicator 58 which via conduit 62,, selective switch 64 and conduit 65 supplies an impulse to regulator 61 in response to steam flow variations.

The other source is the pressure difference indicator 66. This device evaluates and compares the pressure existing at header 20 and that existing at header 34. In accordance with the invention a minimum pressure drop must be maintained in the fluid heating tubes throughout the load range 32 in order to establish a minimum velocity of the working fluid through these tubes to prevent overheating. Any deviation from this desired pressure drop is evaluated by device 66 and transmitted to regulator 61 via conduit 67, selective switch 64, conduit 65 to open valve 54 in response to a decrease in pressure drop and to close valve 54 in response to an increase. in pressure drop through the water heating tubes 32.

Similarly if the selective switch 64 is moved to the left hand position contacting contact point 63 as herein described, an impulse is received by regulator 61 from steam flow indicator 58 which will close valve 54 in response to an increase in stream flow and open valve 54 in response. to a decrease in steam flow.

In this manner, and in accordance with the invention, the recirculating working fluid quantity complements the feed water quantity so as to maintain a rate of flow of average velocity or total overall pressure drop in fluid heating tubes 32, that is constant throughout any selected steaming load range of the modified once-through steam generator, as earlier herein described in connection with FIGS. 2, 3, 4 and 5.

Description of the FIG. 7 Embodiment Another application of my invention is illustratively shown in FIG. 7. There regulation of the recirculated working fluid is accomplished by controlling the speed of the recirculating pump in response to feed pump speed. Also a steam generator is here shown which operates at subcritical pressure, i.e., at pressures below 3206 psi. abs.

As in the embodiment of FIG. 6, water flows from tank 14 via feed water pump 10 through water heating tubes 8 32 to header 34. The steam evaporated in tubes 32 is separated from the residual water in a separator 35 and flows via header 38 through superheater 40' into outlet header 42. A steam pipe 44 serves to transport the steam to a point of use such as a turbine (not shown).

The supply of feed water is regulated in response to steam load requirements by controlling the speed of feed pump motor 12 by means of impulses transmitted through steam flow indicator 58 and conduit 59, to speed adjusting means 67. The fuel supply to burner 26 is controlled in the manner earlier described in connection with FIG. 6.

The recirculating circuit returning water from outlet header 34 to inlet header 20 of the water heating tubes 32 comprises pipe 53 conducting water, collected in separator 35, to the inlet of recirculating pump 50, pipe 52 delivering the recirculated water from the outlet of pump Sil to header 2! via a throttle valve 54.

Circulation pump St is driven by a motor 51 the speed of which is controlled by a speed adjusting device 68 responding to impulses received from a speed responsive device 69, which in turn is governed by and sensitive to variations in revolutions occurring at the feed water pump shaft 76 and sensed by device 71. In accordance with my invention an increase of shaft revolutions transiitted by device 71 to speed responsive device 69 will be translated by device 69 into impulses that cause speed adjusting device 68 to reduce the speed of motor 51 and decrease the delivery of circulating pump 50. A decrease of the feed pump speed as sensed by device 71 will have the opposite effect, namely, to increase the circulation pump speed. In this manner the amount of water recirculated through water heating tubes 32 can be adjusted for various steam loads in response to feed water flow reflected by feed pump speed, so as to maintain a desired minimum pressure drop through Water heating tubes 32 throughout a given boiler load range.

Description of the FIG. 8 Embodiment In FIG. 8 an embodiment of my invention is shown in which the amount of working fluid recirculated through fluid heating tubes 32 is regulated in response to the turbogenerator power output. The steam generator shown is of the forced flow modified once-through type operating in the supercritical pressure range. Fuel input is regulated in response to steam pressure variation as earlier herein described in connection with FIG. 6. Feed water supply is regulated by controlling the feed water regulating valve 22 in response to steam flow and steam temperature, as will hereinafter be described in detail.

Chemically treated water is taken from the water supply tank 14- by feed water pump it) by way of pipe 16 and delivered under pressure via pipe 18 to inlet header 20. From header 20 the working fluid passes through heating tubes 32 to outlet header 34, thence to superheater inlet header 38 through superheater 49, outlet header 42, steam pipe 44 to a turbo-generator comprising turbine 74 driving an electric generator 75. The steam from the turbine is exhausted into condenser 76, the condensate passing through condenser pump 77, feed water heaters 78 back into water reservoir 14 by way of pipe line 79, thus completing the steam power cycle.

The feed water flow to tubes 32 is regulated by the regulating valve 22 which is designed and operated in such a manner that flow is a direct function of valve position. An anticipating impulse from steam flow is provided by the pressure dilterential across a flow nozzle 89 in the steam piping leaving header '34. Thus the pressure differential is received by pressure differential receiver 82 and an impulse is transmitted to a valve regulating device 83 for regulating feed valve 22. The final impulse is obtained through a thermostat 84 indicating the temperature of the steam as it leaves tubes 32. This final impulse as it is received by valve regulator 83 serves to correct the feed water flow to maintain the desired steam temperature. Corrected by this temperature the 9 feed water flow is matched with the heat absorption in tubes 32.

In order to obtain the desired control characteristics of the feedwater regulating valve 22 the pressure drop across the valve is automatically maintained constant by the pressure difference regulating valve 72 which is actuated by pressure difference receiver 85. The pressure difference valve 72 is interconnected with the speed regulator 86 of feed pump turbine 87 to assure adequate feed water supply and maintain the valve position within the desired capacity range.

The amount of working fluid recirculated through tubes 32 is shown in FIG. 8 to be regulated in two ways. First an electric load impulse is transmitted from electric output indicator 88 via selective switch 87 to valve regulator 61 in order to open valve 54 in response to a decrease in load, and to close valve 54 in response to an increase in load. Or, when selective switch 87 is placed in the right hand position the impulse received from load indicating device 88 is transmitted to the speed regulator 90 of the circulating pump turbine 89, so that a decrease in load causes an increase in the speed of pump and an increase of the load causes a decrease in the speed of pump 50.

In this manner the amount of working fluid recirculated through tubes 32 between headers 29 and 34 is accu rately regulated for different boiler loads as expressed in electric output to complement the feed fluid flow through tubes 32, so that an average velocity or total overall pressure drop therethrough can be maintained at a constant value throughout the entire load range, or any selected load range.

Summary From the foregoing it will be seen that my invention has provided important improvements in the operation of high pressure steam generators of the forced flow oncethrough type, that my invention permits substantial and material reductions in the operating head of the feed pump and in the power requirements theerof; that my invention permits the operation of a modified once-through type steam generator at substantially lower steam loads than those heretofore obtainable; that these advantages can be achieved without thereby increasing the danger of overheating the fluid heating tubes exposed to the hot gases; that my invention permits greater safety and greater speed in starting up the steam generator from zero load without former limitations imposed by possible overheating of tubes; that my invention permits unusual flexibility in controlling the flow velocities through the various sections of the fluid heating cycle to compensate for changing operating conditions due to fluctuations in steam load; and that my invention, by reducing first costs and operating costs including power requirements of the feed pump and related equipment, contributes in a significant and substantial degree to a higher overall power plant efficiency.

Although I have shown electrical systems of control it will become apparent that the same results can be achieved through the medium of other well known control facilities.

Furthermore, although I have shown and described herein specific arrangements of control systems in which only a single fluid heating section has been equipped with my controllable recirculation circuit and mode of regulation, it is understood that such a circuit and control can be applied to other heating sections of the oncethrough forced flow system. Such sections may be located in the liquid phase or in the vapor phase of the steam cycle, or partially in the liquid phase and partially in the vapor phase. Or they may constitute radiant heating surface located in or adjacent the furnace chamber, or convection heating surface placed in a gas passage located outside of the furnace chamber, or form any combination of heating sections being part of a forced flow once-through vapor generating and vapor heating cycle.

The particular controls and arrangements here disclosed are therefore illustrative rather than restrictive and my inventive improvements accordingly have broad utility and are capable of wide application.

' I claim:

1. In a forced flow modified once-through type vapor generator having means defining a continuous path of hot combustion gases, in combination a first heat exchanger, 21 second heat exchanger and a third heat exchanger arranged in heat exchange relation with said gases for heating a working fluid and generating vapor at normal operating pressure; first pump means for exerting a force upon a first quantity of working fluid for series flow through a flow path including said first heat exchanger, said second heat exchanger, and said third heat exchanger in the order named and at normal operating pressure at or above a predetermined minimum safe velocity; control means for progressively increasing or decreasing said first quantity as the generator operating load progressively increases or decreases, respectively; second pump means for forcing in mixed relation with said first quantity a second quantity of working fluid through a portion of said flow path including at least one of said second or third heat exchangers at least throughout a major and lower portion of the entire operating load range of said vapor generator and at normal operating pressure, said second pump means being effective in withdrawing said second quantity from the mixed flow path at a location downstream of said one heat exchanger and returning it to the flow path at a location upstream of said one heat exchanger; and control means for directly and progressively decreasing or increasing said second fluid quantity as the generator load progressively increases or decreases, respectively, throughout said major and lower load range portion, whereby to permit reduction in the force exerted by said first pump means upon said first quantity when operating at maximum generator load from the force that would be necessary Without said second quantity flow, while maintaining at least said minimum permissible safe velocity in said one heat exchanger throughout the load range of said vapor generator.

2. In a forced flow modified once-through type steam boiler having a first stage, a second stage and a third stage of fluid heating tubes exposed to hot gases for the generation of steam at a normal operating pressure, the combination of feed pump means for establishing a continuous series flow of working fluid through said first stage, second stage and third stage fluid heating tubes in the order named, and at or above a predetermined safe velocity, recirculating conduit means operatively connected to said second stage heating tubes and including recirculating pump means for establishing recirculation of the said working fluid through said second stage fluid heating tubes and said recirculating conduit means throughout a lower and major portion of the normal operating range and at normal operating pressure; a control device responsive to variations in pressure drop occurring within a predetermined section of said heating tubes; a device for directly regulating the amount of working fluid being recirculated through said heating tubes; and activating means responsive to said control device upon a decrease in pressure drop for operating said regulatingdevice to increase the amount of working fluid being recirculated and upon an increase in pressure drop for operating said regulating device to decrease the amount of working fluid being recirculated, whereby to permit a reduction of said pressure drop and the pressure head of said feed pump means at a load above said lower major load range portion, below that which would be required without recirculation, While maintaining at least said predetermined safe velocity in said second stage fluid heating tubes throughout the load range of said steam boiler.

3. In a steam boiler of the forced flow modified oncethrough type described having water heating tubes, furnace wall tubes and superheating tubes exposed to hot gases for the generation of steam at a normal operating pressure, the combination of feed pump means for establishing a continuous series flow of working fluid through said boiler including said Water heating tubes, said furnace wall tubes and said superheating tubes in the order named and at or above a predetermined safe velocity, recirculating conduit means operatively connected to said furnace wall tubes and including recirculating pump means for establishing recirculation of the working fluid through said furnace wall tubes and said recirculating conduit means throughout a lower and major portion of the normal operating range and at normal operating pressure, a control device responsive to variations in feed pump speed, means for directly regulating the amount of working fluid being recirculated through said furnace wall tubes, and activating means responsive to said control device and operative to activate said regulating device to increase the amount of working fluid being recirculated in response to a decrease in feed pump speed and to decrease the amount of working fluid being recirculated in response to an increase in feed pump speed, whereby to permit a reduction of the pressure head of said feed pump means at a load above said lower major load ra-nge portion, below the pressure head that would be required without recirculation, while maintaining at least said minimum safe velocity in said furnace wall throughout the load range of said steam boiler.

4. An organization as defined in claim 3 wherein said means for regulating the amount of working fluid being recirculated comprises a variable speed motor driving said recirculating pump means, and said activating means comprises a speed adjusting device operatively connected to said motor and responsive to said control device to increases the speed of said motor upon adecrease in feed pump speed and to decrease the speed of said motor upon an increase in feed pump speed.

5. In a forced flow modified once-through type steam power plant having a steam generating boiler including fluid heating tubes exposed to hot gases for producing steam at a normal operating pressure, said fluid heating tubes comprising water heating tubes, furnace wall tubes and superheating tubes, the combination of a steam receiving turbo-generator for generating electricity, feed pump means for establishing a continuous and series flow of working fluid through said boiler water heating tubes, furnace wall tubes and superheating tubes in the order named and at or above a predetermined safe velocity, recirculating conduit means operatively connected to said furnace wall tubes and including recirculating pump means for establishing recirculation of the working fluid through said furnace wall tubes and said recirculating conduit means throughout a lower and major portion of the normal operating range and at normal operating pressure, a control device responsive to variations in electric output on said turbo-generator, a device for directly regulating the amount of working fluid being recirculated through said furnace wall tubes, activating means responsive to said control device upon decrease in electric output for operating said regulating device to increase the amount of working fluid being recirculated and upon an increase in electric output for operating said regulating device to decrease the amount of working fluid being recirculated, whereby to permit a reduction of the pressure head of said feed pump means at a load above said lower major load range portion, below the pressure head that would be required without recirculation, while maintaining at least said predetermined safe velocity in said furnace wall tubes throughout the load range of said steam power plant.

6. An organization as defined in claim 5 wherein said regulating device comprises a throttle valve operatively connected to said recirculating conduit means for regulating the fluid flow therethrough in response to variations in electric output as aforesaid.

7. An organization as defined in claim 5 wherein said regulating device comprises a speed governing device operatively connected to said recirculating pump means for increasing or decreasing the amount of working fluid being recirculated in response to variations in electric output as aforesaid.

8. In a vapor generating and vapor heating apparatus of the forced flow modified once-through type having working fluid heating tubes exposed to hot gases for the generation of vapor at a normal operating pressure, said fluid heating tubes comprising liquid heating tubes, furnace wall tubes and vapor heating tubes, the combination of means for establishing a flow of working medium through said heating tubes in series in the order named at or above a predetermined safe velocity, said means including a feed conduit and feed pump means producing a pressure head suitable for overcoming the flow resistance of said fluid flowing through said liquid heating tubes, said furnace wall tubes and said vapor heating tubes, a recirculating conduit operatively connected to said feed conduit and to the outlet of said furnace wall tubes; recirculating pump means coacting with said recirculating conduit for establishing a recirculation of the working fluid through said furnace wall tubes and said recirculating conduit at normal operating pressures and throughout a lower and major portion of the normal operating range; a control device responsive to variations in output load on said apparatus; a device for directly regulating the amount of working fluid being recirculated through said furnace wall tubes; and activating means responsive to said control device and effective upon a decrease in load for operating said regulating device throughout a lower and major portion of said load range to continually increase within said lower and major load range portion the amount of working fluid being recirculated and upon an increase in load for operating said regulating device to continually decrease within said lower and major load range portion the amount of working fluid being recirculated, whereby to permit a reduction of said flow re sistance that must be overcome by the pressure head of said feed pump at a load above said lower major load range portion, below the flow resistance that would be produced without recirculation, while maintaining at least said predetermined safe velocity in said furnace tubes throughout the load range of said apparatus.

9. An organization in accordance with claim 8 wherein said device for regulating the amount of working fluid being recirculated comprises a throttle valve operatively connected to said recirculating conduit means for r gulating said recirculated fluid flow in response to variations in boiler load as aforesaid.

10. In a forced flow modified once-through type vapor generating and vapor heating apparatus having working fluid heating tubes exposed to hot gases, said heating tubes comprising a fluid preheating tube section, a furnace tube section and a vapor heating tube section, in combination: feed pump means for flowing said working fluid in series through said fluid preheating tubes, said furnace tubes and said vapor heating tubes at or above a predetermined minimum safe velocity; fluid recirculating means including a recirculating conduit operatively connected to the inlet and to the outlet of said furnace tubes; a recirculating pump operative in establishing recirculation of the Working fluid through said furnace tubes and said recirculating conduit at normal operating pressure and throughout a lower and major portion of the normal operating range; a control device responsive to variations indicative of the output load of said apparatus; means for directly regulating the amount of working fluid being recirculated through said furnace tubes; means responsive to said control device and effective for operating said 75 regulating means so that upon a decrease in load to continuously increase within said lower and major load range portion the amount of working fluid being recirculated and upon an increase in load to continuously decrease within said lower and major load range portion the amount of working fluid being recirculated, whereby to permit reduction of the over-all pressure drop of said fluid that must be overcome by said feed pump means when operating at the higher loads above said lower and major load range portion, from the pressure drop that would be required without such recirculation, while maintaining at least said predetermined minimum safe velocity in said furnace tubes through the load range of said apparatus.

11. In the operation of a forced flow modified oncethrough type vapor generator having a fluid preheating zone, a furnace wall fluid heating zone and a vapor heating zone, the method of producing vapor at a maximum design pressure and temperature from a predetermined low load up through full load, comprising the steps of forcing the working medium through a continuous path including in series said fluid preheating zone, said furnace wall fluid heating zone and said vapor heating zone in an amount substantially proportional to a load and at a pressure to produce said maximum design pressure at the distal end of said path, imparting heat to said medium at said zones and along said path and in sufiicient quantity to heat said medium to its desired temperature upon reaching said distal end, dividing the flow of said medium into a plurality of confined streams at one of said furnace wall fluid heating and vapor heating zones and which streams are arranged to delineate a furnace chamber, burning a fuel and passing the hot gases thus produced through said chamber, imparting heat from said gases to said confined streams, throughout at least the lower and major portion of the load range and at normal operating pressure, withdrawing from said path a portion of the working medium after its traversal of said one zone and reintroducing the same into said path upstream of said one zone, continuously eflecting such recirculation of working medium from said predetermined low load up through a given load below the maximum including a lower and major portion of the normal operating load range and at normal operating pressure; recirculating a maximum quantity at said predetermined low load and progressively decreasing the recirculation as the load is progressively increased throughout at least said lower and major portion of load range to thereby maintain the flow velocity in said confined streams above a predetermined value and permit a reduction in the pressure drop developed across said path at loads above said given load from the pressure drop that would be necessary without suc recirculation.

12. The method of operating a forced fiow modified once-through type working fluid vapor generator through out the entire load range thereof, said generator having a fluid preheating zone, a furnace wall fluid heating zone and a vapor heating zone, the steps comprising: exerting a first force upon a first quantity of the working fluid at normal operating pressure for flowing said first quantity in heat absorbing relation with hot combustion gases through a continuous path comprising in series said fluid preheating zone, said furnace Wall fluid heating zone and said vapor heating zone; progressively increasing or decreasing said first quantity as the load progressively increases or decreases, respectively; exerting a second force upon a second quantity of the working fluid at said normal operating pressure and at least throughout a lower and major portion of said load range for flowing said second quantity in mixed relation with said first quantity and in heat absorbing relation with said hot combustion gases through a continuous path including one of said first furnace wall fluid heating and vapor heating zones; returning said second quantity from a point of said continuous path downstream of said one heating zone to a point of said continuous path upstream of said one heating zone; and progressively increasing or decreasing said second quantity as the load progressively decreases or increases, respectively, throughout said lower and major load portion, whereby to permit a reduction in said first force exerted upon said first quantity when operating at maximum generator load from the force that would be necessary Without said second quantity flow, while maintaining at least a minimum permissible velocity of the Working fluid in said one heating zone throughout the entire load range of said vapor generator.

References Cited in the file of this patent UNITED STATES PATENTS 2,199,214 Vorkauf Apr. 30, 1940 2,217,635 Bailey et al Oct. 8, 1940 2,255,612 Dickey Sept. 9, 1941 2 257,749 La Mont Oct. 7, 1941 2,324,513 Junkins July 20, 1943 2,405,573 Frisch Aug. 13, 19 6 2,656,823 Hillier Oct. 27, 1953 FOREIGN PATENTS 509,746 Great Britain July 20, 1939 719,753 Great Britain Dec. 8, 1954 

